1

COMBINED REMOVAL OF DIESEL SOOT PARTICULATES AND NOx OVER

CeO2-ZrO2 MIXED OXIDES.

I. Atribak, A. Bueno-López*, and A. García-García.

Department of Inorganic Chemistry, University of Alicante,Ap.99 E-03080, Alicante, Spain.

(atribak@ua.es, agus@ua.es, a.garcia@ua.es)

Abstract

CeO2 and Ce-Zr mixed oxides with different Ce:Zr ratio have been prepared,

characterised by Raman spectroscopy, XRD, TEM, N2 adsorption at -196ºC and H2-TPR,

and tested for soot oxidation under NOx/O2. Among the different mixed oxides,

Ce0.76Zr0.24O2 provided the best results. Ce0.76Zr0.24O2 presents a higher activity than pure

CeO2 for soot oxidation by NOx/O2 when both catalysts are calcined at 500ºC (soot

oxidation rates at 500ºC are 14.9 and 11.4 gsoot/s, respectively), and the catalytic activity

of CeO2 decays significantly with calcination temperature (from 500 to 1000ºC) while

Ce0.76Zr0.24O2 presents enhanced thermal stability at temperatures as high as 1000ºC. In

addition, Ce0.76Zr0.24O2 catalyses more efficiently than CeO2 the reduction of NOx by soot

around 500ºC, therefore contributing to the decrease of the NOx emission level. The

catalytic activity of CeO2 and Ce0.76Zr0.24O2 for soot oxidation by NOx/O2 depends on the

textural properties (BET area; crystallite size), but other properties of the oxides, like redox

behaviour and/or enhanced lattice oxygen mobility, also play a significant role.

Keywords: catalysed soot oxidation; ceria; Ce-Zr mixed oxide; solid solution; soot; NOx;

Diesel pollution control;

* Corresponding authorTel.: +34 965 90 34 00 (2226)Fax: +34 965 90 34 54

Manuscript

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1.- Introduction.

Ceria is an important material in the framework of the three-way catalysts (TWCs)

which are used in gasoline automobile catalytic converters for the treatment of exhaust

gases [1, 2]. One of the key functions of this material is the ability of cerium to switch

between the Ce4+ and Ce3+ oxidation states and to incorporate more or less oxygen into

their crystal structure depending on various parameters such as the gaseous atmosphere

with which they are in contact, temperature and pressure [3]. Therefore, early on, ceria was

given a main role as oxygen storage component in order to extend the three-way window

on the lean side of stoichiometry by acting as a sink for gas-phase oxygen during rich-to-

lean transients in air-to-fuel ratio [1-3]. On the other hand, the oxygen storage component

could also promote oxidation of reductants, like CO, during lean-to-rich transients [3]. The

TWCs formulation has undergone significant advances during the past 20 years, mainly

employing ceria in solid solution with other metal oxides, most notably zirconia [1, 4]. The

advanced TWC formulations are capable of much higher temperature operation than their

predecessors and have dramatically improved long-term emissions performance [2, 4].

It has been established that addition of zirconium to ceria to form a mixed oxide

solution greatly enhances the reducibility of Ce4+ in the catalyst material, which has

generated considerable interest in the Ce-Zr system [5-8]. The energetics of the Ce4+/Ce3+

reduction step and the corresponding formation of oxygen vacancies are likely to be

involved [9]. Indeed, it is clear that fundamental solid state properties such as the precise

role of structural defects and dopants, the nature of redox reactions to create electronic

species (both within the bulk and at the surfaces), as well as the mechanism of oxygen

migration, are crucial to the greater understanding of these important materials.

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The unique features of oxygen storage capacity have made Ce-Zr binary oxides

important in numerous catalytic processes, besides TWCs, also CO oxidation [10], light

hydrocarbon combustion [11, 12] and VOC oxidation [13]. The key steps of these reactions

are the supply of oxygen by the readily reducible mixed oxide and its re-oxidation by

oxygen. The Ce-Zr mixed oxides are better catalysts than bare CeO2 because the partial

substitution of Ce4+ (ionic radii = 0.97 Å) with Zr4+ (ionic radii = 0.84 Å) leads to the

deformation of the lattice, improving its oxygen storage capacity, redox properties and

thermal resistance [5-8, 14, 15].

It has been also recently reported [16] the important role of the active oxygen

generated by CeO2 in the catalysed oxidation of soot by O2, which seems to be of interest

for the utilization of this oxide as catalyst for the regeneration of soot traps fitted in the

exhaust of Diesel engines [17, 18]. It has been also shown that ceria doped with Zr [19], La

[20, 21] or Pr [21] results in more active catalysts for soot oxidation by O2, but scarce

information has been reported regarding soot combustion under NOx/O2 mixtures catalysed

by Ce-Zr mixed oxides.

The catalytic behaviour of CeO2 for soot oxidation in O2 is not the same than that in

the presence of NOx. In a previous study, the catalytic activity of pure TiO2, ZrO2, and

CeO2 for soot oxidation under NOx+O2 mixtures was compared [22], concluding that the

best activity of CeO2 is related to its capacity to accelerate the NO conversion to NO2 (TiO2

and ZrO2 do not catalyse this reaction), NO2 being much more oxidant than NO and O2

[23]. Rare earth-doped ceria catalysts (Rare earth = Sm, Y, La or Pr) have been also studied

for soot oxidation by NOx/O2, concluding that La and Pr doping enhances ceria activity and

stability, while Sm and Y do not provide such benefit [24, 25].

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It has been reported [22] that CeO2 catalyses the reduction of NOx by soot,

removing both Diesel pollutants - soot and NOx - simultaneously. The simultaneous

removal of NOx and soot was previously studied by Pisarello et al. [26] with catalysts

containing Co, K and/or Ba supported on MgO, La2O3 and CeO2, concluding that CeO2

supplies the oxygen necessary for the redox mechanism that takes place during the reaction.

The scope of this paper is to study the catalytic performance of Ce-Zr mixed oxides

for the removal of soot under simulated Diesel exhaust conditions in a gas flow containing

O2 and NOx. Special attention is paid to the simultaneous removal of NOx. The catalytic

activity of the Ce-Zr mixed oxides is compared with that of pure CeO2 and the thermal

stability of the pure and mixed oxides is studied in detail since the thermal stability is a key

requirement for the application of these materials in a real soot trap.

2.- Experimental

2.1. Catalyst preparation

Ce-Zr mixed oxides with different metal ratio were prepared by using a co-

precipitation route. The required amounts of ZrO(NO3)2·6H2O and/or Ce(NO3)3·6H2O

(supplied by Aldrich) were dissolved in water and the hydroxides were precipitated by

dropping an ammonia solution to keep the pH about 9. The precipitates were dried at 90ºC

in air overnight and calcined in air for 3 hours at different temperatures in the range 500-

1000ºC. Pure CeO2 and ZrO2 were also prepared following the same procedure.

CeO2 and ZrO2 pure oxides are denoted by CeO2-T and ZrO2-T, respectively, and

the Ce-Zr mixed oxides are denoted by CexZr1-xO2-T, (formal composition, 0<x<1) where x

is the molar fraction of CeO2 in the mixed oxides and T is the calcination temperature.

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2.2. Catalytic tests

The catalytic tests were performed in a tubular quartz reactor coupled to specific

NDIR-UV gas analysers for CO, CO2, NO, NO2 and O2 monitoring. 20 mg of soot and 80

mg of the selected catalyst were mixed in the so-called loose contact conditions [27] and

diluted with SiC to avoid pressure drop and favour heat transfer. The gas mixture used

contained 500 ppm NOx, 5% O2 and balance N2, and the gas flow was fixed at 500 ml/min

(GHSV = 30000 h-1).

Two types of experiments were carried out:

i) Temperature Programmed Reactions: the gas mixture was fed to the

reactor, which was at room temperature, and then the temperature was

increased from room temperature until 700ºC at 10ºC/min.

ii) Isothermal Reactions at 500ºC: the temperature was raised from room

temperature until 500ºC, maintaining the soot-catalyst mixture in N2 flow,

and then the N2 flow was replaced by the reaction gas mixture. These

experiments were conducted until total conversion of soot.

The model soot used is a carbon black from Degussa (Printex U), with < 0.1 % ash

content, 5 % volatile matter, 92.2 % C, 0.6 % H, 0.2 % N and 0.4 % S.

Soot conversion profiles were determined from CO and CO2 evolved and the

selectivity of the different catalysts towards CO emission was determined with the

equation:

CO/COx (%) = 100·CO/(CO +CO2)

Blank experiments were performed under the described experimental conditions but

without soot, that is, only using catalyst.

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2.3. Characterization techniques

The textural properties were determined by multi-point N2 adsorption at -196°C

using an automatic Autosorb-6B (Quantachrome equipment). Data were treated in

accordance with the BET method. The samples were previously degassed for 4 hours at

250°C under vacuum.

A JOEL (JEM-2010) microscope was used to obtain TEM images of the catalysts.

Few droplets of an ultrasonically dispersed suspension of each catalyst in ethanol were

placed in a grid and dried at ambient conditions for TEM characterisation.

Raman spectra of the catalysts were recorded in a Bruker RFS 100/S Fourier

Transform Raman Spectrometer with a variable power Nd:YAG laser source (1064 nm). 64

scans at 85 mW laser power (70 mW on the sample) were recorded and no heating of the

sample was observed under these conditions.

X-ray difractograms of the catalysts were recorded in a Bruker D8 advance

diffractometer, using CuK radiation. Spectra were registered between 10 and 80º (2) with

a step of 0.02º and a time per step of 3 seconds. The average crystal size (D) of the catalysts

was determined using the equations of Scherrer and Williamson-Hall:

θ·cosβ

λK·D Scherrer’s equation

where is the wavelength of the radiation used ( = 0.15418 nm for CuK), is the full

width at half maximum of the diffraction peak considered (111), K is a shape factor, which

is taken as 0.9 (1 being a perfect sphere) and is the diffraction angle at which the peak

appears.

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The estimation of crystal size of doped oxides presents some problems, because the

introduction of foreign cations within the lattice deforms the structure and affects the

values. The Williamson–Hall’s equation separates the effects of size and strain in the

crystals, and is more convenient for the estimation of crystal size of mixed oxides:

·cos

)·sin·(4

·cos

·9.0βββ StrainSizeTotal d

d

D

Williamson–Hall’s equation

where Total is the full width half maximum of the XRD peak and d is the difference of the

d spacing corresponding to a typical peak. A plot of Total·cos against 4·sin yields the

average crystal size from the intercept value. In spite of the improvement introduced by the

Williamson–Hall’s equation with regard to the Scherrer’s equation, the crystal sizes

estimated for the Ce-Zr mixed oxides prepared in this study must be considered as semi-

quantitative values, since some of the data obtained are not fully consistent with the BET

areas estimated by N2 adsorption.

The redox behaviour of the catalysts was examined by H2-Temperature-

Programmed Reduction (H2-TPR) in a Micromeritics Pulse ChemiSorb 2705 device

consisting of a tubular quartz reactor coupled to a TCD detector in order to monitor H2

consumption. The reducing gas used was 5% H2 in He. The temperature range explored

was from room temperature to 900°C, and the heating rate 10°C/min.

3. Results and discussion.

3.1 Temperature programmed reactions: Soot oxidation.

On Figure 1, the catalytic activity of two series of Ce-Zr mixed oxides, calcined at

500 and 1000 ºC respectively, is compared. The temperature required to convert 50% of

soot (T50%) in each experiment is plotted as a function of the Zr molar fraction of the

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catalysts used. The T50% temperature for the uncatalysed reaction was 606 ºC under the

experimental conditions used, and most of the samples decreased this temperature. Among

the catalysts calcined at 500ºC, CeO2-500 and Ce0.76Zr0.24O2-500 presented the best

performance, and increasing the Zr loading above this molar fraction has a negative effect

on the activity due to the lower amount of Ce available. This is consistent with the fact that

the couple Ce3+/4+ is the responsible of the catalytic activity for soot oxidation of CeO2-

based catalysts. The same activity trend has been reported [19] for CexZr1-xO2-catalysed

soot combustion in air (0x1.0 composition; catalysts calcined at 500ºC; soot and catalyst

in tight contact), where the highest activity was obtained with cerium-rich mixtures (with x

= 1.0 and x = 0.75). The arguments provided [19] to explain these results were: i) cerium-

rich samples present higher availability of surface Ce4+ sites and, ii) the better ability of

these materials (solid solutions) to donate its oxygen for soot oxidation. The highest

catalytic activity of CexZr1-xO2 mixed oxides with x ~ 0.75, in comparison with higher

zirconium loading, has been also reported for the selective catalytic reduction of NOx by

hydrocarbons [28].

The activity of the catalysts decreased when they were calcined at 1000 ºC (Figure

1), and the degree of thermal deactivation depends on their composition: (i) pure CeO2 and

pure ZrO2 became inactive when calcined at 1000ºC, (ii) Ce0.76Zr0.24O2 suffered a partial

decrease in activity after calcination at 1000ºC, and (iii) the Ce-Zr mixtures with Zr molar

fraction higher than 0.24 calcined at 1000ºC maintained the poor activity of the counterpart

catalysts calcined at 500ºC. The thermal stability of CeO2 is a critical point for its

application in catalysis, as occurred in TWCs where the low stability of the pure CeO2 used

in early formulations obligated to develop advanced CeO2-based materials with enhanced

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thermal resistance [2]. The effect of foreign cation doping on the thermal stability of CeO2

was extensively investigated by Pijolat et al. [29], concluding that, among the different

cations investigated (Th4+, Zr4+, Si4+, La3+, Y3+, Sc3+, Al3+, Ca2+, and Mg2+), those with

ionic radii smaller than that of Ce4+ effectively stabilised the CeO2 against sintering. This

observation is consistent with the improved thermal resistance of Ce0.76Zr0.24O2 in

comparison to pure CeO2.

An additional feature of the soot oxidation catalysts that must be analysed together

with the decrease in soot oxidation temperature is the production of CO2 and/or CO as soot

gasification product, CO2 being the desired gas product due to the high toxicity of CO. A

general feature of carbon combustion reactions is that the production of CO increases and

the production of CO2 diminishes with temperature. Therefore, on one hand, whether a

catalyst decreases the soot combustion temperature, CO2 formation is favoured and, on the

other hand, CeO2-based catalysts are effective to catalyse the oxidation of CO to CO2 [2].

Both properties of the catalysts determine the amount of CO and CO2 yielded during the

catalysed soot oxidation reactions.

On Figure 2, the percentage of CO evolved during the experiments performed with

the series of catalysts calcined at 500 and 1000ºC is included. The uncatalysed reaction

yields 65% CO, and all the catalysts decreased this value with the only exception of CeO2-

1000. The best results were obtained with CeO2-500 and with Ce-Zr mixtures with Zr

molar fraction equal or lower than 0.46. The calcination temperature of the Ce-Zr mixtures

had a minor effect on CO selectivity.

As a summary of the results included on Figures 1 and 2, it can be concluded that

the best formulation obtained among those tested is Ce0.76Zr0.24O2. This material has the

same activity and selectivity for CO2 formation than pure CeO2 when both catalysts are

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calcined at 500ºC, but shows enhanced thermal stability, maintaining part of its activity for

soot oxidation and selectivity for CO2 formation after calcinations at a temperature as high

as 1000ºC. This conclusion is in agreement with that reached by Aneggi et al. [19] for

CeO2- and Ce0.75Zr0.25O2-catalysed soot combustion in air (with catalysts calcined at 500

and 800ºC), who concluded that the main difference between pure ceria and cerium–

zirconium solid solution is related to the stability after calcination.

The effect of calcination temperature on the catalytic activity of CeO2, ZrO2 and

Ce0.76Zr0.24O2 was studied in more detail within the range of 500-1000ºC, and the results

obtained corresponding to activity and selectivity towards CO2 formation are compiled on

Figures 3 and 4, respectively. In all the series of catalysts, the catalytic activity for soot

oxidation (Figure 3) decreases with calcination temperature. As expected, ZrO2 presents

very poor activity, regardless the calcination temperature [22]. The activity of CeO2 and

Ce0.76Zr0.24O2 is similar when catalysts are calcined at 500ºC but not at higher calcination

temperatures, Ce0.76Zr0.24O2 catalysts being, as a general trend, more active than their

counterpart CeO2 catalysts. The activity of CeO2 decreases quite monotonically from 500 to

1000ºC, CeO2-1000 being not active at all. On the contrary, the activity of Ce0.76Zr0.24O2 for

soot oxidation suffers a certain decrease between 500 and 600 ºC (T50% increases from

521 to 543ºC, respectively) but is not further modified significantly until 900ºC, indicating

very high thermal stability within the range 600-900ºC. The activity of Ce0.76Zr0.24O2 only

suffered a further decrease between 900 and 1000ºC, but it still maintains part of its

activity.

The effect of calcination temperature on the selectivity towards CO2 formation

depends significantly on the catalyst formulation (Figure 4). Regardless the calcination

temperature, Ce0.76Zr0.24O2 yields a low CO percentage (136 %) and ZrO2 a high CO

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percentage (5312 %). CO formation during CeO2-catalysed soot oxidation suffers a little

increase with catalysts calcined between 500 and 600ºC but raises drastically between 600

and 800ºC, almost reaching at this temperature the CO percentage yielded during the

uncatalysed reaction. These results confirm that, among the different formulations studied,

Ce0.76Zr0.24O2 is the best one considering its activity for soot oxidation, high thermal

stability and high selectivity towards CO2 formation as soot oxidation product.

3.2. Temperature programmed reactions: NOx elimination.

The elimination of NOx during the Temperature Programmed Reactions also

depended on the catalyst used, as observed on Figure 5a for catalysts calcined at 500ºC.

This type of profiles was previously studied in detail for pure CeO2 catalysts [22].

Considering, as an example, the NOx elimination profile obtained with Ce0.76Zr0.24O2 (see

Figure 5a), three different processes can be distinguished between 225 and 700ºC, that is,

the maximum NOx elimination level is reached at 400ºC, and two shoulders appear at

higher temperatures, around 500 and 600 ºC, respectively. This profile suggests that three

different NOx elimination pathways are taking place. The shape of the NOx elimination

curves depends on the catalyst used, in other words, the relative importance of each NOx

elimination pathway is different for each catalyst. The NOx elimination profiles are

explained as follows:

NOx elimination around 400ºC and lower temperatures: the elimination of NOx is

mainly attributed to NOx chemisorption on the catalysts, as supported by the blank

experiments (without soot) included on Figure 6a. NOx chemisorption on Ce-Zr

mixtures (Figure 5a) around 400ºC decreases by increasing the Zr molar fraction, since

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NOx chemisorption occurs on Ce but not on Zr [22]. The NOx elimination levels

reached at 400ºC by Ce0.76Zr0.24O2-500 and CeO2-500 (Figure 5a) are slightly different,

while the NOx chemisorption in both catalysts observed in blank experiments (Figure

6a) is the same. This suggests a certain contribution of another NOx elimination

pathway (NOx reduction by soot), in addition to NOx chemisorption.

NOx elimination around 500ºC: the elimination of NOx around this temperature is

partially attributed to the catalysed soot-NOx reaction, and NOx is consumed due to

reduction by soot. This is the most interesting NOx elimination pathway, since allows

the simultaneous abatement of NOx and soot by reaction of both pollutants to each

other. The formation of N2O, as reaction product, was not detected in additional

experiments followed by gas chromatography [22]. O2 is also converted around 500ºC

due to soot combustion, as observed on Figure 5b. As mentioned in the previous

section, the catalytic activity for soot oxidation of the series of catalysts calcined at

500ºC decreases when the Zr molar fraction increases (Figure 1), Ce0.76Zr0.24O2-500

being the most active Ce-Zr mixture.

NOx elimination around 600ºC: The elimination of NOx around this temperature is

mainly attributed to the uncatalysed soot-NOx reaction, occurring along with the

reaction soot-O2 (see Figure 5b). As observed on Figure 5a, this is the only NOx

elimination pathway for the least active catalysts (ZrO2-500, Ce0.16Zr0.84O2-500, and

Ce0.34Zr0.66O2-500).

A key factor influencing the catalysed soot oxidation reactions in the presence of

NOx is the NO2 production by the different catalysts. On Figure 6b, the NO2 percentage (on

the basis of NO+NO2) is plotted as a function of temperature for blank experiments. Above

275ºC, Ce0.76Zr0.24O2-500 and CeO2-500 catalyse the oxidation of NO to NO2, reaching a

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maximum NO2 level corresponding to the thermodynamic equilibrium at 450ºC, and

decreasing at higher temperature following thermodynamics. On the contrary, the catalysts

calcined at 1000ºC are not effective to convert NO into NO2, which is consistent with the

lower activity of the samples Ce0.76Zr0.24O2-1000 and CeO2-1000 for soot oxidation in

comparison to the counterpart samples calcined at 500ºC. It is well established [30] that the

catalytic activity for soot oxidation of ceria-based catalysts under NOx mixtures is related

to their ability to accelerate NO2 production, NO2 being more oxidant than NO and O2.

The effect of calcination temperature of CeO2 and Ce0.76Zr0.24O2 on their NOx

elimination capacity was also studied, and the curves obtained during the corresponding

Temperature Programmed Reactions are included on Figures 7a and 7b, respectively. As

shown on Figure 7a, the amount of NOx removed below 500ºC is seen decreased with

calcination temperature, existing a dramatic decrease from CeO2-600 to CeO2-700. The

removal of NOx through the uncatalysed reaction (around 600ºC) is the main NOx

elimination pathway for CeO2 calcined at 800°C and higher temperatures.

The behaviour of Ce0.76Zr0.24O2-series (Figure 7b) is different to that of CeO2-series

(Figure 7a). Ce0.76Zr0.24O2 suffers a gradual decrease in their NOx removal capacity and

only the sample Ce0.76Zr0.24O2-1000 exhibits a NOx profile ascribed only to the uncatalysed

NOx-soot reaction. Once more, NOx elimination profiles allow concluding that

Ce0.76Zr0.24O2 is the best formulation among those studied, since it presents the best

performance among the different catalysts tested and shows enhanced thermal stability with

regard to bare CeO2.

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3.3 Isothermal reactions at 500ºC.

As described in the previous sections, the results of the temperature programmed

reactions indicated that some of the catalysts tested are able to promote the soot-NOx

reaction around 500ºC, therefore allowing the simultaneous removal of both NOx and soot.

Considering the best performance of Ce0.76Zr0.24O2, its catalytic activity has been tested

under isothermal conditions at 500ºC, and it has been compared with that of bare CeO2. In

both cases, catalysts calcined at 500 and 1000ºC have been tested. The soot oxidation rates,

expressed per gram of soot remaining in the reactor, are included on Figure 8a and the

corresponding NOx elimination profiles on Figure 8b. Additionally, the average soot

oxidation rates estimated from isothermal experiments at 500ºC are compiled on Table 1 as

well as the percentage of CO evolved during the experiments and the time required for 75%

soot conversion into CO+CO2. It is worth mentioning that the uncatalysed soot oxidation at

500ºC did not take place under these experimental conditions.

Ce0.76Zr0.24O2-500 is the most active catalyst for soot oxidation (Figure 8a) and NOx

reduction (Figure 8b). The soot oxidation rate during the Ce0.76Zr0.24O2-500-catalysed

experiment rose with soot conversion until about 25 % of conversion approximately, which

can be explained by an increase of the number of actives sites for oxygen chemisorption on

the soot surface as a consequence of its progressive oxidation [31, 32]. After a certain

conversion, the steady state is reached and a constant rate is maintained. As shown on

Table 1, the main reaction product is CO2, mainly for catalysts calcined at 500°C, with a

very low CO emission (according to TPR results). The reduction of NOx (Figure 8b)

decreased progressively with the soot conversion, as expected since less reductant is

available, and NOx elimination was null after about 75% soot conversion. However, a low

amount of NOx was further eliminated once soot is about consumed, and the NOx

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elimination profiles slightly rose at the end of the experiment. This phenomenon was

attributed to the chemisorption of NOx on the catalyst [22]. If soot is available (in Figure 8,

below 75% soot conversion for Ce0.76Zr0.24O2-500) the reaction of NOx (mainly NO2) with

soot is preferable to the storage of NOx on the catalyst, but once soot is not available (above

75% soot conversion), NOx chemisorption on the catalyst takes place being the only

possible NOx elimination pathway.

The calcination temperature of the catalyst diminishes their activity and the soot

oxidation rate (Figure 8a) in steady state conditions is about 3 times lower with the catalyst

Ce0.76Zr0.24O2-1000 than with Ce0.76Zr0.24O2-500. Under our experimental conditions, 75%

of the soot used in the experiments was burnt off after 19 and 62 minutes with

Ce0.76Zr0.24O2-500 and Ce0.76Zr0.24O2-1000, respectively. The practical implication of this

result is that the regeneration of a DPF loaded with Ce0.76Zr0.24O2-1000 would take about 3

times longer that the regeneration of the same filter loaded with Ce0.76Zr0.24O2-500. The

activity of CeO2-500 also decreased after calcinations at 1000ºC, but in this case the effect

of the calcinations temperature was drastic. 172 minutes were required by CeO2-1000 to

convert 75% of the soot used in the experiment while 24 minutes were required by CeO2-

500. Since in a real filter thermal aging of the catalyst can occur, thermally stable active

phases are required and Ce0.76Zr0.24O2 is preferable to pure CeO2. In addition, the activity of

CeO2-500 for soot oxidation and NOx reduction under isothermal conditions at 500ºC is

lower than that of Ce0.76Zr0.24O2-500 (Figure 8). These differences were not detected in the

transient conditions of the Temperature Programmed Reactions (Figures 1 and 3),

evidencing the importance of performing experiments under isothermal conditions, which

also provide a more realistic picture of the catalytic behaviour under real conditions.

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3.4. Study of the catalytic activity decay due to thermal aging.

The thermal deactivation of CeO2 and Ce0.76Zr0.24O2 has been studied, and their

BET surface areas are represented as a function of the calcination temperature on Figure 9.

The BET surface areas of the samples calcined at 500ºC are about the same value

(65 m2/g), and the surface areas of all the oxides decrease significantly with calcination

temperature. Ce0.76Zr0.24O2 presents much better thermal resistance than pure CeO2 due to

the stabilizing role of Zr. In this type of mixed oxides, the BET value is related to the

external surface area of the particles, and high surface area means small particle size and

vice versa. The TEM pictures included on Figure 10 are in agreement with this argument

and also the crystal sizes determined from XRD by Scherrer and Williamson Hall’s

equations (Table 2). CeO2-500 and Ce0.76Zr0.24O2-500 (Figures 10a and 10c, respectively)

appear in TEM pictures as a quite homogeneous agglomeration of small particles with a

particle size around 10-20 nm in both cases, which is consistent with the crystal sizes

determined from the Williamson Hall’s equations (22 and 21 nm, respectively). After

calcination at 1000ºC, CeO2 particles grow due to thermal sintering forming particles larger

than 100 nm (Figure 10 b) while Ce0.76Zr0.24O2-1000 maintains smaller sizes of around 20-

30 nm (Figure 10 d). On this line, the Williamson Hall’s equation (Table 2) estimates

crystal sizes of 107 and 65 nm for CeO2-1000 and Ce0.76Zr0.24O2-1000, respectively.

The structural characterization of CeO2 and Ce0.76Zr0.24O2 samples was carried out

by XRD and Raman spectroscopy (Figures 11 and 12 respectively). All the diffractograms

included on Figures 11a and 11b contain the main reflections typical of a fluorite-structured

material with a fcc unit cell at 28.5, 33.1, 47.6, and 56.5 º, corresponding to the (111),

17

(200), (220) and (311) planes [33]. Evidences of phase segregation were not observed by

XRD for the Ce-Zr mixed oxide (Figure 11b), since ZrO2 characteristic peaks did not

appear. However, a certain degree of inhomogeneous distribution of cerium and zirconium

cannot be ruled out, as it will be next discussed in the context of H2-TPR characterization.

Zr incorporation into the fluorite structure of CeO2 caused the lattice deformation,

and considering a same calcination temperature, the intensity of the CeO2 peaks is higher

than that of the counterpart Ce0.76Zr0.24O2 mixed oxide. This is a consequence of the better

arrangement of the atoms into the framework of pure CeO2 and to the decrease of the

number of lattice defects. The effect of calcination temperature is clearly envisaged on

Figure 11a. As calcination temperature increases the peaks of CeO2 become narrower. This

is related to an increase in crystal size, which is consistent with BET and TEM

characterization, and also with the negative effect of temperature in the catalytic activity of

CeO2. The same effect of temperature is observed in the patterns included on Figure 11b

but in a lower extent, in agreement with the higher thermal stability of the Ce-Zr mixed

oxide.

Raman characterization supports the conclusions of XRD. All the CeO2 and

Ce0.76Zr0.24O2 samples (Figures 12a and 12b) present the typical structure of CeO2 with the

main band at 460 cm-1 attributed to the only allowed Raman mode (F2g) of a fluorite-type

structure [34, 35]. The Raman spectra of these fluorite-type oxide structures are dominated

by oxygen lattice vibrations and are sensitive to the crystalline symmetry [36]. The

presence of Zr4+ into the CeO2 lattice deforms the structure and the intensity of the fluorite-

characteristic peak decreases significantly, as deduced by comparison of Figure 12a with

Figure 12b. It has been reported that this deformation favours oxygen mobility affecting the

redox behaviour of the material [37]. The calcination temperature also affects the

18

arrangement of atoms in the CeO2 lattice (Figure 12a), and the intensity of the Raman peak

increases with calcination temperature as a consequence of the better arrangement of atoms.

Raman characterization also provides evidences of the improved thermal stability of the

Ce-Zr mixed oxide. As shown on Figure 12b, the intensities of the signals of Ce0.76Zr0.24O2

calcined between 500 and 900ºC are about the same, and increase slightly when calcined at

1000ºC. The shift of the Raman signal towards lower energies that occurs in Ce0.76Zr0.24O2

calcined between 500 and 900ºC with regard to Ce0.76Zr0.24O2-1000 is a sign of the

improvement of oxygen mobility, which could be related to the presence of oxygen

vacancies [38].

The redox properties of selected samples were investigated by H2-TPR, and the H2

consumption profiles obtained have been plotted on Figure 13. H2 consumption must be

attributed to the reduction of Ce4+ to Ce3+, since Zr4+ is a non-reducible cation. It is

generally accepted [39] that two peaks characterise the reduction profile of pure CeO2. The

first peak, centred at around 500 ºC in the profile of CeO2-500, is attributed to the reduction

of the uppermost layers of Ce4+ and the second peak, centred at 800 ºC, is originated by the

reduction of the bulk. The H2-consumption profile of Ce0.76Zr0.24O2-500 also shows this

shape, but the peak intensity ratio (surface/bulk reduction) is higher for the mixed oxide.

This suggests enhanced oxygen mobility within the Ce0.76Zr0.24O2 lattice in comparison

with the bare CeO2. On the other hand, a certain degree of inhomogeneous distribution of

cerium and zirconium can be also inferred. A broad H2-consumption band would be

expected for a true Ce-Zr solid solution, where the surface and bulk reduction would occur

concurrently, but not a clear distinction between surface and bulk reduction. Wu et al. [40]

also reported a bimodal H2-TPR profile for a Ce0.5Zr0.5O2 sample which was ascribed to a

possible “shell/core structure” concordant with a heterogeneous surface elemental

19

distribution. These authors reported a surface Ce/Zr ratio of 1.3, which is higher than the

nominal ratio (1.0) for this sample. The results of XPS analysis performed with our

Ce0.76Zr0.24O2 mixed oxide also showed a Ce/Zr ratio of 5.0, well above the value of 3.2

expected for this nominal composition. On this line, Nagai et al [41,42] also assessed (by

means of the XAFS technique) the existence of Ce- and Zr-rich domains in a Ce0.5Zr0.5O2

solid solution prepared by co-precipitation and subsequent calcination at 500°C”.

The profile of CeO2-1000 only contains the bulk reduction peak due to the very low

BET surface area of this sample. The drastic effect of the calcination temperature on the

surface redox properties of CeO2 is not so obvious in Ce0.76Zr0.24O2. The H2-consumption

profile of Ce0.76Zr0.24O2-1000 shows a broad band instead of two well-defined peaks, and

the onset temperature of this band is consistent to a surface reduction process. The

generally accepted suggestion to explain this type of profile is that the surface and bulk

reduction occurs concurrently [39], that is, there is not a clear distinction between surface

and bulk peaks because oxygen located within the bulk comes to the surface when surface

oxygen is consumed. This type of profiles is characteristic of materials with good oxygen

mobility.

As a summary, the characterization of CeO2 and Ce0.76Zr0.24O2 allows concluding

that both materials present fluorite structure, and that Ce-Zr incorporates Zr4+ cations into

the CeO2 framework. This incorporation of foreign cations enhances the thermal resistance

of CeO2, diminishing thermal sintering. As a consequence of this improved thermal

resistance, Ce0.76Zr0.24O2 calcined at high temperature presents higher BET surface area and

improved redox properties than pure CeO2.

It is not easy to distinguish between the relative contribution of structural properties

(BET area; particle size) and other properties like redox behaviour or lattice oxygen

20

mobility, in the catalytic activity towards soot combustion of CeO2-based pure and mixed

oxides. In order to get insight into this, the temperature for 50% soot conversion obtained

from Temperature Programmed Reactions performed with the different CeO2 and

Ce0.76Zr0.24O2 samples has been plotted as a function of their BET surface area on Figure

14. A relationship between both parameters is clearly observed, which allows concluding

that the catalytic activity of pure CeO2 and that of the mixed oxide Ce0.76Zr0.24O2 depends

on its surface area. A relationship between surface area and catalytic activity for soot

oxidation in temperature programmed oxidations by O2, with soot and catalyst in tight

contact, has been also reported for the catalyst Ce0.95Fe0.05O1.975 [19]. This catalyst was

calcined at different temperatures in the range 500-750ºC, thus reaching surface areas in the

range 92-10 m2/g, respectively. A linear correlation between surface area and activity was

obtained for surface areas below 40 m2/g, while for larger area the variation in activity was

not so relevant. In the present study, this threshold is not observed, probably because the

soot and catalyst are in loose contact instead of tight. For experiments in loose contact, we

also expect a threshold in surface area values above which the catalytic activity does not

longer improves, but this limit seems to be above the maximum surface area reached in the

current study (67m2/g).

In spite of the relationship between catalyst surface area and T50% values obtained

in temperature programmed reactions (Figure 14), the isothermal reactions performed at

500ºC indicated that Ce0.76Zr0.24O2-500 presents better activity than CeO2-500 (and both

samples present the same BET area). These differences could be explained by the improved

redox properties, deduced from H2-TPR (Figure 13), and/or enhanced lattice oxygen

mobility, deduced from Raman spectroscopy (Figure 12), of the mixed oxide. In

conclusion, the structural properties of CeO2-based pure and mixed oxides play an

21

important role on their catalytic activity for soot oxidation but other properties like redox

behaviour and/or enhanced lattice oxygen mobility, also affect their performance.

4.- Conclusions.

As a summary of the current study, it can be concluded that the catalytic activity of

CeO2 for Diesel-exhaust purification can be significantly improved by doping CeO2 with

Zr4+, Ce0.76Zr0.24O2 being the best formulation among those prepared and tested. This Ce-Zr

mixed oxide presents a slightly higher activity than bare CeO2 for soot oxidation by NOx/O2

when both catalysts are calcined at 500ºC (soot oxidation rates at 500ºC are 14.9 and 11.4

gsoot/s, respectively), and both catalysts presents the same selectivity for CO2 formation as

soot oxidation product. However, Ce0.76Zr0.24O2 shows enhanced thermal stability in

comparison to pure CeO2 (as deduced form XRD, Raman, TEM, N2 adsorption and H2-

TPR characterization), maintaining part of its activity for soot oxidation and selectivity

towards CO2 formation after calcination at temperatures as high as 1000ºC.

In addition, Ce0.76Zr0.24O2 catalyses more efficiently than CeO2 the reduction of NOx

by soot around 500ºC, therefore contributing to the decrease of the NOx emission level.

The catalytic activity of CeO2 and Ce0.76Zr0.24O2 for soot oxidation by NOx/O2

depends on their textural properties (BET area; crystallite size), but other properties of the

oxides, like redox behaviour and/or enhanced lattice oxygen mobility, also play a

significant role.

22

Acknowledgements

The authors thank the financial support of the project CTQ2005-01358 of the

Spanish Ministry of Education and Science and ABL the contract funded by the Ramon y

Cajal Program and the Generalidat Valenciana.

23

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26

FIGURE CAPTIONS

Figure 1. Effect of the Ce:Zr ratio on soot combustion.

Figure 2. Effect of the Ce:Zr ratio on CO emission.

Figure 3. Effect of the calcination temperature of catalyst on soot combustion.

Figure 4. Effect of the calcination temperature of catalyst on CO emission.

Figure 5. Effect of the Ce:Zr ratio on: (a) NOx elimination and (b) O2 elimination (catalysts calcined at 500ºC).

Figure 6. Blank experiments (without soot): (a) NOx elimination and (b) NO2 formation.

Figure 7. Effect of the calcination temperature of catalyst on NOx elimination: (a) CeO2-series and (b) Ce0.76Zr0.24O2-series.

Figure 8. Isothermal reactions at 500ºC: (a) soot oxidation rate and (b) NOx elimination. (The units of soot oxidation rate are mg of soot converted per second and gram of soot remaining in the reactor)

Figure 9. BET surface areas in terms of calcination temperature of catalysts.

Figure 10. TEM images : a) CeO2-500, b) CeO2-1000, c) Ce0.76Zr0.24O2-500, d) Ce0.76Zr0.24O2-1000.

Figure 11. XRD characterisation of catalysts: (a) CeO2-series and (b) Ce0.76Zr0.24O2-series.

Figure 12. Raman characterisation of catalysts: (a) CeO2-series and (b) Ce0.76Zr0.24O2-series.

Figure 13. H2 consumption profiles during H2-TPR for selected catalysts.

Figura 14. T50% parameter versus BET surface area of catalysts (CeO2-series and Ce0.76Zr0.24O2-series).

27

Table 1. Data estimated from isothermal reactions at 500ºC.

Table 2. Crystal sizes determined from XRD.

CatalystCO/COx

(%)Time for 75% soot conversion (min)

Average soot oxidation rate (gsoot/s)

CeO2-500 2.1 24 11.4CeO2-1000 11.3 172 1.2Ce0.76Zr0.24O2-500 3.5 19 14.9Ce0.76Zr0.24O2-1000 36.8 62 4.1

Crystal size (nm) with Williamson-Hall’s equation

Crystal size (nm) with Scherrer’s

equationCeO2-500 22 14CeO2-600 22 19CeO2-700 46 32CeO2-800 63 38CeO2-900 116 58CeO2-1000 107 55Ce0.76Zr0.24O2-500 10 11Ce0.76Zr0.24O2-600 21 13Ce0.76Zr0.24O2-700 20 9Ce0.76Zr0.24O2-800 17 12Ce0.76Zr0.24O2-900 19 13Ce0.76Zr0.24O2-1000 65 22

28

500

525

550

575

600

0 0.2 0.4 0.6 0.8 1

Zr (molar fraction)

T50

% (

ºC)

No catalyst

Catalyst calcined at 500 ºC

Catalyst calcined at 1000 ºC

Figure 1

29

0

25

50

75

0 0.2 0.4 0.6 0.8 1

Zr (molar fraction)

CO

/CO

x (

%)

No catalyst

Catalyst calcined at 500 ºC

Catalyst calcined at 1000 ºC

Figure 2

30

500

525

550

575

600

500 600 700 800 900 1000Calcination temperature (ºC)

T50

% (

ºC)

No catalyst

ZrO2

CeO2

Ce0.76Zr0.24O2

Figure 3

31

0

25

50

75

500 600 700 800 900 1000Calcination temperature (ºC)

CO

/CO

x (

%)

No catalyst

ZrO2

CeO2

Ce0.76Zr0.24O2

Figure 4

32

0

3

6

9

12

15

18

21

225 325 425 525 625

Temperature (ºC)

NO

x el

imin

atio

n (%

)

No catalyst

(a)

ZrO2

CeO2

Ce0.76Zr0.24O2

Ce0.34Zr0.66O2

Ce0.54Zr0.46O2

Ce0.16Zr0.84O2

0

3

6

9

12

15

225 325 425 525 625

Temperature (ºC)

O2

elim

inat

ion

(%)

ZrO2CeO2

Ce0.76Zr0.24O2

Ce0.54Zr0.46O2

No catalyst

(b) Ce0.34Zr0.66O2

Ce0.16Zr0.84O2

Figure 5

33

0

3

6

9

12

15

18

21

225 325 425 525 625

Temperature (ºC)

NO

x el

imin

atio

n (%

)

CeO2-500

Ce0.76Zr0.24O2-500

CeO2-1000

Ce0.76Zr0.24O2-1000

(a)

0

10

20

30

40

50

225 325 425 525 625

Temperature (ºC)

NO

2 (%

)

(b)

CeO2-500

Ce0.76Zr0.24O2-500

CeO2-1000Ce0.76Zr0.24O2-1000

Thermodynamic equilibrium

No catalyst

Figure 6

34

0

3

6

9

12

15

18

21

225 325 425 525 625

Temperature (ºC)

NO

x el

imin

atio

n (%

)

1000

500 600

700

800

No catalyst

900(a)

0

3

6

9

12

15

18

21

225 325 425 525 625

Temperature (ºC)

NO

x el

imin

atio

n (%

)

1000

600

700

800 No catalyst

900

(b)

500

Figure 7

35

0.0

0.4

0.8

1.2

1.6

0 25 50 75 100Soot conversion (%)

Soo

t oxi

datio

n ra

te(m

g soo

t/s·g

rem

aini

ng s

oot) (a) Ce0.76Zr0.24O2-500

CeO2-1000

Ce0.76Zr0.24O2-1000

CeO2-500

0

25

50

75

100

0 25 50 75 100Soot conversion (%)

NO

x el

imin

atio

n (%

)

Ce0.76Zr0.24O2-500

CeO2-500

Ce0.76Zr0.24O2-1000

CeO2-1000

(b)

Figure 8

36

0

10

20

30

40

50

60

70

500 600 700 800 900 1000

Calcination temperature (ºC)

BE

T (

m2 /g

)

ZrO2

CeO2

Ce0.76Zr0.24O2

Figure 9

37

Figure 10

(c)

50 nm

(a)

50 nm

(d)

(b)

200 nm50 nm

38

20 30 40 50 602 (º)

Inte

nsit

y (a

rb.)

(a)

CeO2-500

CeO2-600

CeO2-700

CeO2-800

CeO2-900

CeO2-1000

(111)

(200)

(220)

(311)

(222)

20 30 40 50 602 (º)

Inte

nsit

y (a

rb.)

(b)

Ce0.76Zr0.24O2-1000

(111)

(200) (220) (311)

Ce0.76Zr0.24O2-900

Ce0.76Zr0.24O2-800

Ce0.76Zr0.24O2-700

Ce0.76Zr0.24O2-600

Ce0.76Zr0.24O2-500

Figure 11

39

350 400 450 500 550 600

Raman shift (cm-1)

Inte

nsity

(ar

b.)

CeO2-1000

CeO2-900

CeO2-800

CeO2-700

CeO2-600

CeO2-500

(a)

350 400 450 500 550 600

Raman shift (cm-1)

Inte

nsity

(ar

b.)

Ce0.76Zr0.24O2-900

(b)

Ce0.76Zr0.24O2-1000

Ce0.76Zr0.24O2-800

Ce0.76Zr0.24O2-700

Ce0.76Zr0.24O2-600Ce0.76Zr0.24O2-500

Figure 12

40

0 200 400 600 800 1000

Temperature (ºC)

H2

cons

umpt

ion

(arb

.)

CeO2-500

CeO2-1000

Ce0.76Zr0.24O2-500

Ce0.76Zr0.24O2-1000

Figure 13

41

500

525

550

575

600

0 10 20 30 40 50 60 70

BET (m2/g)

T50

% (

ºC)

CeO2 series

Ce80Zr20 series

CeO2 series

Ce0.76Zr0.24O2 series

Figure 14